1. Technical Field
The present invention relates to a chemical-mechanical polishing apparatus of a semiconductor wafer and a method for controlling the same, and more particularly to a chemical-mechanical polishing apparatus for performing an endpoint detection (EPD) in a chemical-mechanical polishing process, and a method for controlling the same.
2. Description
An integrated circuit is generally formed on a substrate by a sequential deposition of a conductive layer, a semi-conductive layer, or an insulation layer on a silicon wafer. After each layer is deposited, each layer is etched in order to form a circuit. As a series of layers are sequentially deposited and etched, a peripheral portion or an uppermost face of the substrate (that is, an exposed face of the substrate) becomes uneven. When the substrate has a non-planar face, some problems may occur during a photolithographic step of an integrated circuit fabrication process. Hence, the face of the substrate should be periodically planarized.
A chemical mechanical polishing (CMP) process has been accepted for planarizing the substrate. In the CMP process, the substrate must be mounted on a carrier head or a polishing head. The exposed face of the substrate is disposed so that the substrate corresponds to a rotating polishing pad. The polishing pad may be a standard pad or a fixed-abrasive pad. While the standard pad has a durable rugged face, the fixed-abrasive surface pad includes abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, to the substrate in order to push the substrate against the polishing pad. When the standard pad is applied, a slurry for polishing, including at least one chemically reactive agent and abrasive particles, is supplied to a surface of the polishing pad.
The effectiveness of the CMP process can be measured by a polishing rate of the substrate, an end result of a polished surface of the substrate (that is, an absence of a small-scaled roughness), and a flatness of the polished surface of the substrate (namely, an absence of a large-scaled roughness). The polishing rate, the end result, and the flatness of the substrate are determined according to a combination of the polishing pad and the slurry, a configuration of the carrier head, a relative speed between the substrate and the polishing pad, and a force pressing the substrate against the pad.
In order to examine the effectiveness of polishing tools and the polishing process, a wafer with one or more layers having no patterns (a so-called “blank” wafer) is polished in a tool/process qualification step. After such a wafer is polished, a thickness of a remaining layer is measured at several points of the polished surface of the substrate. The thickness variations of the layer can provide measurements of a wafer surface uniformity, and a relative polishing rate in different regions of the substrate.
One approach to determine the thickness of a substrate layer and the polishing uniformity is to examine the polished substrate after removing the substrate from the polishing apparatus. For example, the substrate is transferred to a metrology station where the thickness of the substrate layer is measured with an ellipsometer. Disadvantageously, such a method requires a lot of time, high cost, and expensive metrology equipment.
One important point for the CMP process is to determine whether or not the polishing process is exactly completed, i.e., whether or not the layer on the substrate layer is planarized to have a desired flatness or thickness. The polishing rate of the layer on the substrate may be varied according to the initial thickness of the layer on the substrate, the composition of the slurry, the material and the condition of the polishing pad, the relative speed between the polishing pad and the substrate, and the force pressing the substrate against the polishing pad. A variation of the polishing rate may cause variations in the time required to reach a polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of the polishing time.
One approach for detecting the polishing endpoint is to remove the substrate from the polishing pad, and then examine the substrate. When the substrate does not meet desired specifications, the substrate is reloaded into the CMP apparatus for a further processing. Alternatively, an examination for the substrate can reveal that an excess amount of the layer has been removed, which renders the substrate unusable. That is, a method is needed for detecting whether or not the desired flatness or thickness had been achieved in-situ.
Several methods have been developed for detecting the polishing endpoint in-situ. Most of those methods involve monitoring parameters related to surface conditions of the substrate, and then detecting the polishing endpoint when the parameters are abruptly changed. For example, when an insulation layer or a dielectric layer is polished to expose a metal layer formed beneath the insulation layer or dielectric layer, a friction coefficient and a reflectivity of the substrate may be changed abruptly when the metal layer is exposed.
In an ideal system where the monitored parameters are abruptly changed at the polishing endpoint, such polishing endpoint detection methods are acceptable. However, as the substrate is polished, the condition of the polishing pad and the composition of the slurry composition may be varied at an interface between the polishing pad and the substrate. Such variations may imitate a condition where the polishing endpoint is reached, or falsely indicate that the underlying metal layer is not exposed. Additionally, such endpoint detection methods may not be effective when a planarization step is performed only, or when an underlying layer and an overlying layer have similar physical properties.
U.S. Pat. No. 6,190,234 (issued to Boguslaw Swedek et al.) discloses an endpoint detection method with light beams of different wavelengths.
In the above-mentioned U.S. Patent, a pair of endpoint detecting devices, which project light beams with different wavelengths to a semiconductor wafer, is provided in order to measure a time to finish a polishing process more accurately and reliably.
According to the above-mentioned U.S. Patent, an initial thickness of a polishing layer can be calculated as a function of a thickness between one peak and another peak of the light beams, a phase difference between the light beams, and one peak and another peak period of the light beams in a reflection rate trace of the polishing layer by light beams with a single wavelength. However, such a method is not accurate, and thus the initial thickness can be more accurately provided by deducting a closeness of one peak and another peak period from the reflection rate trace of light beams with different wavelengths.
However, because the method disclosed in the above-mentioned U.S. Patent should utilize two endpoint detecting devices for calculating an accurate thickness, the method may hardly applied to the conventional CMP apparatus.
Also, according to the U.S. Patent, a complicated processing program is required for controlling the two endpoint detecting devices, for tracing a reflection ratio, and performing a calculation, so the cost of the polishing process may increase.
In order to solve above-mentioned problems it would be desirable to provide a chemical-mechanical polishing apparatus including a single endpoint detecting device, which can measure a thickness more accurately and can be employed in the conventional devices easily by changing software, and a method for controlling the same.
It would also be desirable to provide a method for controlling the above chemical-mechanical polishing apparatus.
In accordance with one aspect of the present invention, there is provided a method for controlling a chemical-mechanical polishing (CMP) apparatus for polishing a layer to be polished, formed on a lower layer on a semiconductor wafer. First, an “endpoint detection light quantity table” of initial light quantities detected by an endpoint detecting means and corresponding initial thicknesses of a layer to be polished, according to a polishing process recipe of the layer to be polished, is prepared. Next, the polishing process recipe of the layer to be polished is stored into a storing means. Then, a light is projected onto the semiconductor wafer, including the layer to be polished, and an initial light quantity reflected from the layer to be polished is detected by the endpoint detecting means. Then, before the polishing process is initiated, a thickness of the layer to be polished is calculated from a detection signal corresponding to the detected initial light quantity by referring to the entries in the “endpoint detection light quantity table.” Before the polishing process begins, the polishing time is calculated as the time to polish the layer from the calculated initial thickness to a desired final thickness. Then, the layer is polished by the CMP apparatus. While the layer is being polished, the remaining polishing time is counted down or decremented. An endpoint is detected when the remaining polishing time is zero and the polishing process is stopped.
In accordance another aspect of the present invention, there is provided a chemical-mechanical polishing apparatus for polishing a layer to be polished formed on a lower layer on a semiconductor wafer. The apparatus includes storing means for storing an endpoint detection light quantity table of initial light quantities detected by an endpoint detecting means and corresponding initial thicknesses of a layer to be polished, in accordance with a polishing process recipe for the layer to be polished. The apparatus also includes inputting means for inputting the polishing process recipe of the layer to be polished into the storing means and endpoint detecting means for detecting a light quantity reflected from the layer to be polished by projecting a light to the semiconductor wafer. the apparatus further includes controlling means for calculating the initial thickness of the layer to be polished from the detected light quantity data with reference to the end point detection light quantity table, for calculating a polishing time to polish the layer to be polished from the calculated initial thickness to a desired final thickness, for detecting an end point by counting down the calculated polishing time while polishing the layer to be polished, and for stopping polishing the layer when the end point is detected.
Beneficially, a CMP apparatus according to one or more aspects of the present invention includes one endpoint detector, and calculates an initial thickness of a layer to be polished accurately by using a light quantity data detected by the one end point detector. In addition, the CMP apparatus controls a polishing process and determines a polishing endpoint automatically by calculating a polishing time by using the calculated initial thickness of the layer to be polished. Therefore, an accurate control of a polishing endpoint can be achieved by simply adding a program to a controller of a conventional CMP apparatus, and conditions and an efficiency of the operation can be improved.